“A vortex tube is a device, which produces cooling at one end and heating at other end simultaneously without having any moving part if high pressure air is available.”
George Ranque in France in 1981, observed low temperatures in the rotating flow of air in cyclone separators. Based on this observation, he deviced an arrangement known as the Ranque tube or Vortex tube. In 1945, a German Engineer Prof. Hirsch further developed this. It was announced that a device is developed using compressed air discharged hot and cold stream simultaneously at its two ends. Hartnett and Eckert gave a theoretical explanation of this behaviour. The performance of the tube depends upon:
a)Air Parameters b)Tube Parameters
Timothy of I.I.T. Bombay obtained a drop of 780 C with inlet air at 8 bar and 300 K. Hinge and Naganagoudar of I.I.T. Bombay were able to increase a drop to 830 C. Parulekar developed a “short vortex tube”. Its performance is not dependent on inaccurate workmanship or by a poor surface finish.
WORKING OF VORTEX TUBE
Fig.1.Counter flow vortex tube.
Fig.2. Uniflow vortex tube.
Fig.1. Shows a simple vortex tube. In this tube, the outside air, duly compressed and cooled in the heat exchanger expands through a nozzle. The nozzle, which is fitted tangentially to the pipe that is shown in fig.
The tube can be made either counter flow or uniflow. The counter flow is shown in Fig.1. A nozzle arranged tangentially at one end of a tube is supplied with compressed air. The tube at that end is partially closed by diaphragm with a central hole approximately half the tube diameter.
At the other end of the tube, a valve restrict the exist of air. A stream of cold air leaves through the orifice when the valve is partly open. While, the stream of warm air leaves through the valve. On setting of the valve, the temperature of the cold air stream and the flow rate can be changed. When the valve is fully open or close, the following are the results.
(a) Valve fully open. All air comes out through the orifice. There is no reduction of temperature.
(b) When the valve is closed, the flow rate of cold stream increases. When the flow rate is ½ or 1/3 rd of total, the temperature of cold air falls to a minimum value.
(c) The temperature of the cold stream will increase again if the valve is closed further.
Fig.2. Shows the uniform design of a vortex tube. In this design, the cold stream comes out at the same end as the warm stream. The central core of air stream is separated by a special arrangement of orifice and valve. This design is not efficient as counter flow arrangement of a vortex tube.
THEORY OF VORTEX TUBE
The theoretical explanation given by different research workers differs. All research workers have tried to explain how in absence of a mechanical device, a flow of the core of cold air and the hot air around the periphery takes place. The swirl motion is created when the compressed air expands through the nozzle. By injecting smoke, this has been tested experimentally. One of the theory generally accepted is as under:
(i)The compressed air on expanding through the nozzle forms a free vortex due to the particular shape of the nozzle.
(ii)The expanded air then moves a towards the valve end.
(iii)The vortex travels along the periphery of the tube till it reaches the throttle valve due to centrifugal force.
(iv)As the vortex of air reaches near the valve, the K.E. of the air is converted into the pressure energy. This will give point of stagnation.
(v)The stagnation pressure near the valve (up stream region), which is partly closed, is more than the atmospheric pressure, a reverse axial flow starts from the valve end.
(vi)This reversed flow comes in contact with forward moving free vortex. This contact causes the reversed flow to rotate with it.
(vii)There is an energy transfer from the central core to the peripheral layer. Because of this energy transfer, central core is cooled and the outer core will be heated up.
(viii)The outer layer is also heated up because of the friction with the tube and fluid friction. This energy (or heat) is given to the central layer. The energy supply is insignificant compared to pumping of energy from core to the outer layer due to turbulent mixing in the centrifugal flow fields.
(ix)This will result in a flow of cold core surrounding by a hot concentric flow field in the vortex tube.
(x)The centre core passes out through the diaphragm emerges as a cold stream.
(xi)The outer layer passes out through the throttle valve as hot stream.
If the valve is fully open all the air go out at an average temperature slightly lower than the inlet temperature. The diameter of the diaphragm is smaller than the chamber
diameter. The natural tendency of air is to flow towards the hot end, the end having larger diameter. The reverse flow is not possible and hence no cooling will be obtained.
The extract phenomenon of energy transfer is still need detail investigation. The length of hot tube is very important for proper performance of a vortex tube. The heat transfer from the outer to inner layer of air is more if the length of the tube is long. The temperature of core air will increase. The central core is unable to transfer its kinetic energy to the outer layer in sufficient quantity if the length of the tube is less. Thus the kinetic energy is not decreasing sufficiently. This will affect the stagnation temperature and it will also not reduce sufficiently. The usual variation in the length in design is from 20 to 40 times the tube diameter.
Fig.3.Componant of vortex tube.
1. Nozzle: -
The nozzles are of concertinaing type, diverging type or converging – diverging type as per design. An efficient nozzle is designed to have higher velocity, greater mass flow and minimum inlet losses.
The nozzle is used to develop a high tangential velocity in the chamber. Hence, it has to be tangential to the chamber and the losses in the nozzle should be reduced to minimum. The inlet area should be sufficiently large. The length of the passage should be as small as possible. The surface area should be minimum. The nozzle should not be delicate.
Bombay in 1966, studied different forms of nozzles and their effect on the performance of vortex tube.
The nozzles recommended by Merkulov and Parulekar are simple in construction and nozzle tip is rectangular. Merkulov recommended a tip area of 9% of the area of the cross-section of the tube, with the axial width to be twice the radial depth. Timothy found out that a convergent nozzle with both sides curved in opposite direction gave the best results.
It is a cylindrical piece of small thickness. It has a small hole of specific diameter at the centre. Air stream traveling through the core of the hot side is emitted through the diaphragm hole. Larger the diameter of diaphragm, smaller is the pressure drop and larger the ratio of mass of air emerging from the cold side to the mass of air emerging the tube in same time. Smaller diaphragms will give maximum drop. It should be placed as near nozzle as possible. It was found by Alexeyev that a diaphragm with a circular concentric hole gives the best results. He also found that, for maximum temperature drop, the diameter of the diaphragm must lie between (0.3 to 0.4) diameter of the vortex chamber (Dc). Dr Parulekar and Timothy of IIT Bombay recommended and used a diaphragm of diameter 0.5 Dc.
The valve obstructs the flow of air through hot side and it also controls the quantity of hot air through vortex tube. Timothy at I.I.T, Bombay investigated that, 19mm (2/4) needle valve oriented perpendicular to the axis of the tube gave better results.
4.Hot side: -
It is a cylindrical in shape and circular in cross-section. The length of the hot side tube of a vortex tube designed by Dr. Hilsch was bout 50 times the tube diameter. Alexeyev tried hot ends of varying length on 16mm tube and has recommended the length of the tube as 50 times the diameter of the tube diameter same as Dr. Hilsch. Merkulov reduced the length of the hot end. He had inserted a cross in the tube. This flow rectifiers serves as a vortex brake. This will reduce the energy loss due to friction of the hot stream. The optimum length of the hot end by him was (8 to 10) Dc (Tube diameter) Dr. Parulekar invented short vortex tube. His hot end is consisting of three parts.
1. A cylindrical or convergent piece of axial length equal to 6 mm.
2. A divergent truncated cone with axial length equal to 18mm.
3. Cover, which also forms the third part of the hot side is cylindrical and of axial Length equal to 20mm.
It was also found out that the roughness of internal surface does not seriously affect the results.
Timothy at IIT found out that the hot ends with angle of divergence of 6.5 was giving best temperature drop while 10 angles was better for cooling effect.
5. Cold air side: -
This is the other side of the vortex tube. Through this tube, cold air is flowing. No valve is provided in this tube like hot air side tube.
vortex tube temperature at it hot & cold end can be controlled by varying its different parameters such as
1)Insulated and non insulated tube
2)Cold orifice diameter
3)Number of inlet tangential nozzle
4)Diameter of tube.
1) Effect of insulation: -
Fig.4 (a) Cold tube
Fig.4 (b) Hot tube
Temperature and pressure of 290C and 3.5 bar respectively were made for the cold orifice diameter of 0.5D using the single inlet nozzle. The outside surface temperature of the non-insulated hot tube exposing to the surrounding was about 50- 600C, but this reduced to some 320C when using the insulated tube. The temperature differences between the inlet and the tube temperatures against various cold mass fractions are shown in Figures 4a and 4b for the cold and hot tubes, respectively. In the figure, the insulated tube provided higher temperature reduction than the non-insulated one. At a cold mass fraction of 0.345, the highest temperature reductions at the cold tube of the insulated and non-insulated tubes were 19 and 180C, respectively. In addition, in the hot tube the maximum temperature increases of the insulated and non-insulated tubes were found to be 24 and 20 0C respectively, at a cold mass fraction of 0.857. The average temperature
differences between the insulated and non-insulated tubes were in a range of 2 to 30 0C for the cold tub and of 2 to 50Cfor the hot tube. This is because the insulated tube gave less energy loss to the surroundings than the noninsulatedone, causing the higher temperature difference within the tube. For the cold mass fraction ranging from 0.1 to 0.4, the temperature reduction in the cold tube increased, but then decreased for the cold mass fraction over 0.4. In the hot tube, for the cold mass fraction ranging from 0.1 to 0.8, the temperature proportionally increased, but then decreased rapidly for the cold mass fraction above 0.8.
2) Effect of cold orifice diameter: -
The experimental result of temperature drops for different cold orifice diameters ranging from 0.4D to0.9D using one inlet nozzle is depicted in Figure 5. The highest temperature reduction was achieved for all cold orifice diameters when the cold mass fraction was in a range of 0.3 to 0.4. Thus, the maximum temperature drop occurred if the cone valve was adjusted to let the cold mass flow rate leave the cold tube at 30 to 40% of the inlet air. The decrease in temperature in the cold tube was found to be 18, 19, 15, 14, 12, and 10 0C focusing cold orifice diameter of 0.4D, 0.5D, 0.6D, 0.7D, 0.8D and 0.9D at the cold mass fraction of 0.364, 0.375, 0.381, 0.378, 0.373, and 0.372, respectively. Furthermore, the cold orifice diameter of 0.5D yielded the highest potential of temperature reduction in the cold tube than the others. Using the cold orifice diameter ranging from 0.6D to 0.9D (bigger than that of 0.5D) would allow some hot air in vicinity of the tube wall to exit the tube with the cold air. Both the hot air and cold air as flowing out were mixed together which further affected the cold air to have higher temperature. On the other hand, for a small cold orifice diameter of0.4D, it has a higher backpressure and makes the temperature reduction at the cold tube lower.
3) Effect of number of inlet nozzle: -
The effect of the number of inlet nozzles on temperature reduction in the insulated vortex tube was experimentally investigated as shown in Figure 6.The increase in the number of inlet nozzles led to considerable temperature separation. In the figure, the use of 4 inlet nozzles resulted in a higher temperature reduction in the cold tube than that of 1 and 2 inlet nozzles for the cold orifice diameter of 0.5D. The highest temperature drops also were 19, 29 and 30 0C for using1, 2 and 4 nozzles, respectively. Mostly the maximum thermal separation occurred at a cold mass fraction between 0.3 and 0.4.Changing the number of inlet nozzles from 1 to 2 and 4 helped to speed up the flow And to increase the mass flow rate and strong swirl flow into the vortex tube. In addition, this gave rise to higher friction dissipation between the boundary of the flows and a higher momentum transfer from the core region to the wall region. This reduced temperature in the tube core while increased temperature in the tube wall area.
4) Diameter of tube: -
The vortex tube would offer considerably higher backpressures and, therefore, the tangential velocities between the periphery and the core would not differ substantially d for fixed inlet conditions (supply pressure), a very small diameter of to the lower specific volume of air (still high density) while the axial velocities in the core region are high. This would lead to low diffusion of kinetic energy which also means low temperature separation. On the other hand, a very large tube diameter would result in lower overall tangential velocities both in the core and in the periphery region, which would produce low diffusion of mean kinetic energy and also low temperature
A very small cold orifice diameter would give higher back pressure in the vortex tube, resulting, as discussed above, in low temperature separation. On the other hand, a very large cold orifice diameter would tend to draw air directly from the inlet and yield weaker tangential velocities near the inlet region, resulting inflow temperature separation. Similarly, a very small inlet nozzle would give rise to considerable pressure dropping the nozzle itself, leading to low tangential velocities and hence low temperature separation. A very large inlet nozzle would fail to establish proper vortex flow resulting again in low diffusion of kinetic energy and therefore low temperature separation. The inlet nozzle location should be as close as possible to the orifice to yield high tangential
velocities near the orifice. A nozzle location away from the orifice would lead to low tangential velocities near the orifice and hence low temperature separation.
WALL TEMPERATURE DISTRIBUTION
Fig.7 Arrangement of thermocouples along the hot tube wall
Wall temperatures of the hot tube were measured at 15 axial stations equally spaced along the axial distance downstream of the cold orifice plate as can be seen in Figure 7. Figure 8 displays the wall temperature distributions in terms of temperature difference between the wall and the inlet at different cold mass fractions. In a range from x/D=1 to x/D=11, the wall temperature distribution tended to increase, reaching a maximum at x/D=11. After x/D=11, the wall temperature distribution tended to decrease because the hot air near the tube wall region and the cold air in the core region were mixed together due to decaying of swirl flow close to the exit of hot tube. The temperature of the tube wall increased proportionally with the cold mass fraction, except when the cold mass fraction approached unity. It can be observed that at x/D=11, the temperature at the tube wall with 4 inlet nozzles was 780C above the inlet temperature for the cold mass fractions of 0.829.
COMPARISON OF THE PRESENT TUBE WITH THE PREVIOUS INVESTIGATOR
The details of geometries and working conditions of the present vortex tube and the previous work are shown in Table 1. Figure 9 displays the temperature reduction in the cold tube against the cold mass fraction for the present tube, Hilsch’s tube and Guillaume and Jolly’s tube. It is worth noting that the temperature reduction distributions for all tubes show a similar trend despite different tube sizes and inlet conditions. All tubes yield a maximum temperature reduction at the cold mass fraction between 0.3 and 0.4, having a maximum temperature decrease value at about 160C.
A close examination reveals that when using a single tangential inlet nozzle, the temperature drop profile of the present tube is very close to that of Hilsch’s tube
but is slightly different with Guillaume and Jolly’s tube. This comparison has been made to help increase the confidence in the present measurements only.
Table 1. Comparison of the present tube with the previous investigator.
Fig 9. Comparison of temperature reduction in the cold tube
for the present work and previous work.
ADVANTAGES AND DISADVANTAGES
1.Minor leakages are not important since air is used as a working substance.
2.The vortex tube is quite simple in design. The functioning of the vortex tube is also very simple. This is so because; the hot/cold air is controlled with the help of the valve.
3.The vortex tube has no moving parts and hence no maintenance is needed.
4.It is light in weight.
5.It is quite compact.
6.It is possible to cool complicated space with the help of vortex tube.
7.In initial investment it is cheaper.
8.In order to operate this, no expert attendant is required.
Because of very low COP, the vortex tube is not suitable for large capacity refrigeration unit.
1.Cooling of cutting tools. The vortex tube is best suited to cool the cutting tool in a workshop. This is especially true for those materials for which, the use of coolant are not permitted.
2.It is used to cool certain commodities at a low temperature of – 50 C by direct chilling.
3.Air suits. Operators handling toxic gases use these suits. These suits are using during spray painting, maintenance of pressure vessels etc.
4.Vortex tube is also used and suitable for coalmine worker.
5.The turbine blade needs cooling. A vortex tube is used for that.
6.The vortex tube is also used to cool the sample to be cooled and also to maintain the same sample at lower temperature.
7.Since the vortex tube is able deliver cold and hot air simultaneously, it is used when heating and cooling requirements are simultaneous.
8.Vortex tube can be used for shrink fitting where refrigeration is required for a short period.
Pangjet Promvonge and Smith Elasma-ard of Thailand have carried out an experimental study on the temperature separation in the vortex tube and this research finding can be summarized as follows:
1)The increase of the number of inlet nozzles led to higher temperature separation in the vortex tube.
2)Using the tube with insulation to reduce loss to surroundings gave a higher temperature.
3)Separation in the tube than that without insulation around 2-30C for the cold tube and 2-50C for the hot tube.
4)A small cold orifice (d/D=0.4) yielded higher backpressure while a large cold orifice (d/D=0.7, 0.8, and 0.9) allowed high tangential velocities into the cold tube, resulting in lower thermal/energy separation in the tube.